feat(geschichten): add DocumentMultiSelect chip + typeahead component

Mirrors PersonMultiSelect for documents: chip-style multi-select backed by
GET /api/documents/search?q=. Used in the Geschichte editor sidebar to link
referenced documents to a story.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/lib/components/DocumentMultiSelect.svelte

@@ -0,0 +1,147 @@
+<script lang="ts">
+import type { components } from '$lib/generated/api';
+import { m } from '$lib/paraglide/messages.js';
+import { clickOutside } from '$lib/actions/clickOutside';
+import { formatDate } from '$lib/utils/date';
+
+type Document = components['schemas']['Document'];
+type DocumentSearchItem = components['schemas']['DocumentSearchItem'];
+
+interface Props {
+	selectedDocuments?: Document[];
+	placeholder?: string;
+	hiddenInputName?: string;
+}
+
+let {
+	selectedDocuments = $bindable([]),
+	placeholder = m.geschichte_editor_search_document(),
+	hiddenInputName = 'documentIds'
+}: Props = $props();
+
+let searchTerm = $state('');
+let results: Document[] = $state([]);
+let showDropdown = $state(false);
+let loading = $state(false);
+let debounceTimer: ReturnType<typeof setTimeout>;
+let inputEl: HTMLInputElement;
+let dropdownStyle = $state('');
+
+function updateDropdownPosition() {
+	if (!inputEl) return;
+	const rect = inputEl.getBoundingClientRect();
+	dropdownStyle = `position:fixed;top:${rect.bottom + 4}px;left:${rect.left}px;width:${rect.width}px`;
+}
+
+function handleInput() {
+	showDropdown = true;
+	clearTimeout(debounceTimer);
+	debounceTimer = setTimeout(async () => {
+		if (searchTerm.length < 1) {
+			results = [];
+			return;
+		}
+		loading = true;
+		try {
+			const res = await fetch(`/api/documents/search?q=${encodeURIComponent(searchTerm)}&size=10`);
+			if (res.ok) {
+				const body: { items: DocumentSearchItem[] } = await res.json();
+				const docs = body.items.map((it) => it.document);
+				results = docs.filter((d) => !selectedDocuments.some((s) => s.id === d.id));
+			}
+		} catch {
+			results = [];
+		} finally {
+			loading = false;
+		}
+	}, 300);
+}
+
+function selectDocument(doc: Document) {
+	selectedDocuments = [...selectedDocuments, doc];
+	searchTerm = '';
+	showDropdown = false;
+	results = [];
+}
+
+function removeDocument(id: string | undefined) {
+	selectedDocuments = selectedDocuments.filter((d) => d.id !== id);
+}
+
+function formatDocLabel(doc: Document): string {
+	if (doc.documentDate) return `${doc.title} · ${formatDate(doc.documentDate, 'short')}`;
+	return doc.title;
+}
+</script>
+
+<svelte:window onscroll={updateDropdownPosition} onresize={updateDropdownPosition} />
+
+{#each selectedDocuments as doc (doc.id)}
+	<input type="hidden" name={hiddenInputName} value={doc.id} />
+{/each}
+
+<div class="relative" use:clickOutside onclickoutside={() => (showDropdown = false)}>
+	<div
+		class="flex min-h-[38px] flex-wrap gap-2 rounded border border-line bg-surface p-2 shadow-sm focus-within:ring-2 focus-within:ring-focus-ring focus-within:outline-none"
+	>
+		{#each selectedDocuments as doc (doc.id)}
+			<span
+				class="inline-flex items-center gap-1 rounded bg-muted px-2 py-1 text-sm font-medium text-ink"
+			>
+				{formatDocLabel(doc)}
+				<button
+					type="button"
+					onclick={() => removeDocument(doc.id)}
+					class="ml-0.5 text-ink/50 hover:text-red-500 focus:outline-none"
+					aria-label={m.comp_multiselect_remove()}
+				>
+					<svg class="h-3 w-3" fill="none" stroke="currentColor" viewBox="0 0 24 24">
+						<path
+							stroke-linecap="round"
+							stroke-linejoin="round"
+							stroke-width="2"
+							d="M6 18L18 6M6 6l12 12"
+						/>
+					</svg>
+				</button>
+			</span>
+		{/each}
+
+		<input
+			bind:this={inputEl}
+			type="text"
+			autocomplete="off"
+			bind:value={searchTerm}
+			oninput={handleInput}
+			onfocus={() => {
+				updateDropdownPosition();
+				showDropdown = true;
+			}}
+			placeholder={placeholder}
+			class="min-w-[120px] flex-1 border-none bg-transparent p-1 text-sm outline-none focus:ring-0"
+		/>
+	</div>
+
+	{#if showDropdown && (results.length > 0 || loading)}
+		<div
+			style={dropdownStyle}
+			class="ring-opacity-5 z-50 max-h-60 overflow-auto rounded-md bg-surface py-1 text-base shadow-lg ring-1 ring-black sm:text-sm"
+		>
+			{#if loading}
+				<div class="p-2 text-sm text-ink-2">{m.comp_multiselect_loading()}</div>
+			{:else}
+				{#each results as doc (doc.id)}
+					<div
+						class="cursor-pointer py-2 pr-9 pl-3 text-ink select-none hover:bg-muted"
+						onclick={() => selectDocument(doc)}
+						onkeydown={(e) => e.key === 'Enter' && selectDocument(doc)}
+						role="button"
+						tabindex="0"
+					>
+						{formatDocLabel(doc)}
+					</div>
+				{/each}
+			{/if}
+		</div>
+	{/if}
+</div>